SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Restatement and Revision of Previously Reported Consolidated Financial Information
Material costs	$ 79,446	$ 72,899	$ 147,184	$ 139,597
Personnel costs	93,176	83,435	183,129	164,588
Other operating expenses	54,959	51,256	98,802	91,475
Income from operations	36,644	35,726	59,360	57,462
Income before taxes	22,291	28,042	37,230	42,001
Provision for income taxes	8,212	10,656	13,661	15,980
Net income	14,079	17,386	23,569	26,021
Basic earnings (in dollars per share)	$ 0.40	$ 0.51	$ 0.68	$ 0.76
Diluted earnings (in dollars per share)	$ 0.40	$ 0.50	$ 0.67	$ 0.75
Restricted Cash
Restricted cash to eliminate letters of credit obligations	3,120		3,120		3,120
As Previously Reported
Restatement and Revision of Previously Reported Consolidated Financial Information
Material costs				136,998
Personnel costs				167,054
Other operating expenses				93,389
Income from operations				55,682
Income before taxes				40,221
Provision for income taxes				15,280
Net income				$ 24,941
Basic earnings (in dollars per share)				$ 0.73
Diluted earnings (in dollars per share)				$ 0.72